Plague is the deadly flea-borne bacterium you might associate with the Middle Ages, when the “Black Death” killed 50 million people, wiping out most of Europe’s population. But it’s still around today, even in the United States.

In some countries, particularly Madagascar, plague is endemic, and flare-ups cause public health emergencies on an almost annual basis — and right now, an uncontrolled epidemic that’s brewing there appears to be exceptionally worrying.

As November 1, more than 1,800 plague cases have been identified, including 127 deaths. Madagascar typically sees about 400 cases each year, when the disease surfaces from September to April. “This outbreak is unusually severe, and there are still five more months to go before the end of the plague season,” the World Health Organization reported.

Plague has also now spread to coastal cities, “which we have not seen before,” WHO spokesperson Tarik Jašarević told Vox.

Also unlike previous outbreaks, this year’s involves mostly pneumonic plague, a more dangerous form of the disease than the much more common bubonic plague. Pneumonic plague attacks the lungs and spreads from person to person through droplets from coughing, like a cold, while bubonic plague spreads only from fleas to humans. (Sixty-two percent of cases so far were classified as pneumonic plague, according to the WHO.)

“So it’s a worrying situation because it’s in an urban setting and it’s transmissible from human to human,” Jašarević added. “We need to act quickly to trace the contacts of those infected and put people on a preventive treatment.”

Distribution of the total number of plague cases reported in Madagascar, August 1 to October 30, 2017.

There is some potential good news: If you look at the epidemic curve of the outbreak, it seems pulmonary plague cases have recently started to wane. This week, there were only 106 people hospitalized and being treated for plague; the rest had recovered.

But this doesn’t mean the epidemic is over. “We are about three months into the epidemic season,” Jašarević said. “Even if the recent declining trend is confirmed, we cannot rule out the possibility of further spikes in transmission between now and April 2018.”

On October 31, the WHO also warned the risk of more spread within Madagascar remains “very high” and the agency is now helping Madagascar’s neighbors prepare for the worst.

There are nine countries and territories preparing for potential plague outbreaks

Doctors and nurses from the Ministry of Health and officers of the Malagasy Red Cross staff a “health care checkpoint,” trying to inform passengers leaving Antananarivo about plague and to potentially detect suspected cases.

Public health workers found out that many people he was in contact with had come down with plague, and they had spread the disease to others too. Since then, cases have been popping up elsewhere around the country. Fifty-one of Madagascar’s 114 districts have been affected so far.

Because of trade and travel links to Madagascar, there are nine countries and territories that could see plague cases turn up: Comoros, Mauritius, Mozambique, the French regions of Réunion and Mayotte, Seychelles, South Africa, the United Republic of Tanzania, Ethiopia and Kenya. For this reason, the WHO has been beefing up exit screening in Madagascar to make sure people with plague aren’t bringing the bacterium to other countries and helping these nine countries strengthen their epidemiological and laboratory surveillance capabilities.

Overall, though, the WHO thinks the highest risk of plague spread is still within Madagascar, while there’s a “moderate” risk of regional spread to nearby countries. This risk of global spread, however, remains low, and that’s mainly because of the nature of the disease: People with plague get sick (and, if untreated, die) so quickly after infection that they don’t travel as far as say, Ebola, which incubates in the body for days before causing symptoms.

Pneumonic plague is a “public health nightmare”

Plague is caused by the bacterium Yersinia pestis. Wild rodents — chipmunks, mice, squirrels — can carry the bacteria. Fleas feed on the rodents and pick up plague, in turn spreading it to other mammals, including humans, through bites.

When people talk about plague, they’re usually talking about bubonic plague, the type that causes about 80 percent of infections in the US every year. Bubonic plague infects the lymphatic system, causing the lymph nodes to swell up and feel tender. These swellings, known as buboes, typically pop up in the groin, neck, and armpits. A blackish pustule will also arise at the site of the flea bite, and hemorrhaging can occur underneath the skin, causing it to blacken — part of the reason the plague is called “Black Death.”

When bubonic plague is left untreated, it can spread into the lungs to become pneumonic plague, or into the bloodstream to become septicemic plague. These types of plague are more rare — and more dangerous — because unlike bubonic plague, they can be passed directly from person to person.

“A person with pneumonic plague is a public health nightmare,” said Joe Hinnebusch, a plague researcher at the National Institutes of Health.

Still, plague is not as contagious as some other pathogens, like flu or measles, and it requires close contact to spread, said Hinnebusch.

“[Plague is] also fairly easily preventable once an outbreak occurs, and should be fairly easy to stop with simple measures like wearing masks, or isolating people who are sick so they aren’t contacting other people,” he added. Unlike with Ebola, we have a cure for plague in antibiotics.

“There would have to be a total breakdown of public health systems [for the epidemic to continue uncontrolled],” Hinnebusch said.

Still, it’s a highly unusual epidemic right now in Madagascar, with so many pneumonic plague cases so early in the season. And that’s a situation worth keeping an eye on, as infectious diseases expert Crawford Kilian warned on his blog. “Pneumonic plague in a big city is like Ebola in a big city: better to prevent it than suppress it, and prevention now seems out of the question.”

In much of the world, plague is associated with poverty

Houses in Antananarivo, Madagascar, on October 2, as plague spreads rapidly in cities across the country.

As this study in PLOS Medicinereports, poorer countries with bad sanitation and other risky practices can help spread the infection:

Africa is particularly at risk for a number of reasons. Poor rural communities typically live in close proximity to rodents, which are widely hunted and eaten in many plague-endemic areas. Superstitions, lack of money, and distance from health facilities often lead to delays in seeking health care and receiving treatment. The public health system in large parts of Africa is poorly organised and equipped, and political crisis and social disorganisation impede improvements.

So it’s not surprising that on a global scale, plague is most common in places like Madagascar, where the per capita GDP is only $1,500. Plague turned up on the island in 1898 through trade routes, and outbreaks have popped up nearly every year since 1980. (For the past two decades, Madagascar has experienced the highest burden of plague cases in the world.)

But there are other factors that help spread plague: Humans have been encroaching on wildlife areas, putting them into contact with potential carriers of the disease. And this is as true for poorer countries as it is for developed places like the US. As this scientific study on US plague reports, “Plague in New Mexico has increasingly occurred in more affluent areas” — the result of building out suburban and exurban communities in previously underdeveloped areas where plague had been circulating in wild animals.